1. Field of the Invention
The present invention relates to an image processing unit for use in an imaging device such as a digital camera recording image data in monotone mode which performs image correction on digital color image data, an image processing method for digital color image data in an imaging device, and an imaging device incorporating such an image processing unit.
2. Description of the Related Art
Some imaging devices such as a digital camera include a monotone photographing function to generate monotone images. In general, such an imaging device comprises an image sensor as a CCD with a color filter so that color images are generated. In order to generate a monotone image, the imaging device needs to perform monotonizing processing to color images. By way of example, image monotonizing is done by converting RGB data into YUV data and then setting UV data representing a color difference to zero to remove the color of an image. Also, sepia-toned images can be created by setting UV data for all pixels to the same value.
Black tone is one of the key factors to determine the impression of a monotone image. In silver halide photography, black tone representation will differ depending on property of a film developer and photographic paper and there are a neutral pure black tone, a warm black tone with softness added, a cold black tone with sharpness added, and the like.
Needless to say that black tone representation is very important in the monotone photographing with the digital camera. It has been required for the digital camera to realize the same black tone representation as that of the silver halide photography.
In order to meet such a demand, Japanese Laid-open Patent Publication No. 2008-67296 (Reference 1) and No. 2007-166543 (Reference 2) disclose a technique to adjust the black tone (pure black tone, warm black tone, cold black tone) of monotone images according to users' tastes.
Moreover, Japanese Laid-open Patent Publication No. 2004-336386 (Reference 3) discloses a technique to perform three kinds of image processing on full color images to adjust all of brightness, chroma, and hue thereof but perform only a single image processing on monotone (sepia-toned or monochrome) images to adjust brightness thereof.
However, there is a problem that the digital camera with a color filter cannot generate monotone images in the same black tone representation, warm or cold, as the silver-halide camera does, even using the above techniques. Further, realizing warm or cold black tone representation in an image with a digital camera requires a lot of skills and knowledge about photography.